# Kotlin调用Jni的写法

## 一、安装CMake和NDK

​		在android studio的设置中安装Cmake，安装Cmake的时候注意cmake的版本。如果忘记CMake的版本，那么可以通过Android SDK Locatio目录中查看CMake的版本。如下图所示的CMake和NDK。

![image-20230221221558825](E:\技术武器库\技术开发笔记\Android开发笔记\Image\image-20230221221558825.png)

## 二、创建工程

选择一个带 Native C++ 支持的工程，如下图所示：

![image-20230221223742943](E:\技术武器库\技术开发笔记\Android开发笔记\Image\image-20230221223742943.png)

## 三、配置 ndk 目录

​		在文件 local.properties 中加入 ndk 的配置

## 四、目录结构

​		以下文件都是自动生成的

![image-20230221223834839](E:\技术武器库\技术开发笔记\Android开发笔记\Image\image-20230221223834839.png)

## 五、配置模块 build.gradle

```shell
apply plugin: 'com.android.application'
apply plugin: 'kotlin-android'
apply plugin: 'kotlin-android-extensions'

android {
    compileSdkVersion 29
    buildToolsVersion "29.0.2"

    defaultConfig {
        applicationId "com.lujianfei.androidjnicall"
        minSdkVersion 26
        targetSdkVersion 29
        versionCode 1
        versionName "1.0"

        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"

        externalNativeBuild {
            cmake {
                cppFlags ""
            }
        }
    }

    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
    }

    externalNativeBuild {
        cmake {
            path "src/main/cpp/CMakeLists.txt"
            version "3.10.2"
        }
    }
}

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"


    implementation 'androidx.appcompat:appcompat:1.1.0'
    implementation 'androidx.core:core-ktx:1.2.0'
    implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'androidx.test.ext:junit:1.1.1'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.2.0'
}
```

## 六、配置CMakeLists.txt和native-lib.cpp

```
# For more information about using CMake with Android Studio, read the
# documentation: https://d.android.com/studio/projects/add-native-code.html

# Sets the minimum version of CMake required to build the native library.

cmake_minimum_required(VERSION 3.4.1)

# Creates and names a library, sets it as either STATIC
# or SHARED, and provides the relative paths to its source code.
# You can define multiple libraries, and CMake builds them for you.
# Gradle automatically packages shared libraries with your APK.

add_library( # Sets the name of the library.
             native-lib

             # Sets the library as a shared library.
             SHARED

             # Provides a relative path to your source file(s).
             native-lib.cpp )

# Searches for a specified prebuilt library and stores the path as a
# variable. Because CMake includes system libraries in the search path by
# default, you only need to specify the name of the public NDK library
# you want to add. CMake verifies that the library exists before
# completing its build.

find_library( # Sets the name of the path variable.
              log-lib

              # Specifies the name of the NDK library that
              # you want CMake to locate.
              log )

# Specifies libraries CMake should link to your target library. You
# can link multiple libraries, such as libraries you define in this
# build script, prebuilt third-party libraries, or system libraries.

target_link_libraries( # Specifies the target library.
                       native-lib

                       # Links the target library to the log library
                       # included in the NDK.
                       ${log-lib} )
```

```C++
#include <jni.h>
#include <string>

extern "C" JNIEXPORT jstring JNICALL
Java_com_lujianfei_androidjnicall_MainActivity_stringFromJNI(
        JNIEnv* env,
        jobject /* this */) {
    std::string hello = "Hello from C++";
    return env->NewStringUTF(hello.c_str());
}
```

## 七、Activity调用

```kotlin
package com.lujianfei.androidjnicall

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import kotlinx.android.synthetic.main.activity_main.*

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        // Example of a call to a native method
        sample_text.text = stringFromJNI()
    }

    /**
     * A native method that is implemented by the 'native-lib' native library,
     * which is packaged with this application.
     */
    external fun stringFromJNI(): String

    companion object {
        // Used to load the 'native-lib' library on application startup.
        init {
            System.loadLibrary("native-lib")
        }
    }
}
```